The objective of this project will be to understand the regulation of aminoacyl- tRNA synthetases in higher organisms, using rats as our model. This animal has been chosen since isoproterenol administration to rats produces large quantities of proline rich proteins in the rat submandibular glands. Prolyl- tRNA synthetase activity appears to be regulated to allow the synthesis of this protein. The specific aims of this project will be to determine the level of prolyl- tRNA as a fraction of total tRNA under conditions that induce the synthesis of proline-rich proteins; to compare the distribution of isoaccepting tRNA from control and experimental rats; to compare the activities of prolyl- tRNA synthetases in the control and experimental animals; and to follow the above parameters in rats subjected to withdrawal of the drug after isoproterenol treatment.